Help with Baby PIcot Leg Warmers

I am trying to crochet baby leg warmers and the instructions have me make 20 short rows that will become the cuff of the leg warmer. To join the last row to the first row before working in rounds for the body of leg warmer the instructions say:

Join last row with sl sts to back loops of first row.

Do I go into each single crochet stitch of the last row and then into the back loops of the first row to join the rows? That wasn't my understanding of a slip stitch but I can't figure out any other way to join.